Conventionally, audio devices are becoming more digitalized, and players for reproducing music sources are becoming smaller and more portable. However, speaker devices for eventually reproducing sounds require large cabinets so as to sufficiently reproduce sounds in a low frequency region included in music sources. Therefore, speaker devices carried in the small-size or portable players have small-volume cabinets, so that an acoustic stiffness exhibited by the cabinet is large, and therefore, it is difficult to achieve low-frequency sound reproduction to a sufficient extent.
Therefore, a speaker device has been disclosed in which a limit of low-frequency sound reproduction which is determined by the volume of a cabinet is improved (see, for example, Patent Document 1). Hereinafter, the speaker device will be described with reference to FIG. 14. Note that FIG. 14 is a cross-sectional view of a structure of the speaker device.
In FIG. 14, the conventional speaker device comprises a cabinet 101 and a speaker unit 102. The speaker unit 102 has a frame 103, an edge 104, a cone-shaped diaphragm 105, a dust cap 106, a voice coil bobbin 107, a damper 108, a voice coil 109, a magnet 110, a center pole 111, a magnetic plate 112, a movable magnet 113, and a fixed magnet 114.
In FIG. 14, the speaker unit 102 is attached to an opening portion on a front surface of the cabinet 101. The magnet 110 is in the shape of a ring. A back surface of the magnet 110 (a surface of the magnet 110 closer to a back side of the cabinet 101) is fixed to a front side of the center pole 111. Aback surface of the magnetic plate 112 is fixed to a front surface of the magnet 110. The voice coil 109 is wound around an outer circumferential surface of an end portion of the voice coil bobbin 107 closer to the back side of the cabinet 101. The voice coil 109 is inserted in a magnetic gap formed between an outer circumferential surface of a convex of the center pole 111 and an inner circumferential surface of the magnetic plate 112. The frame 103 has a sound hole 103h and is fixed to a front surface of the magnetic plate 112. An outer circumference of the damper 108 is fixed to the frame 103 to support the voice coil bobbin 107. The cone-shaped diaphragm 105 is fixed to an end portion closer to the front surface of the voice coil bobbin 107. The edge 104, fixed to the flame 103, supports an outer circumference of the cone-shaped diaphragm 105. The dust cap 106 is fixed to a center portion of the cone-shaped diaphragm 105. The movable magnet 113 is in the shape of a ring, and an inner circumferential surface of the movable magnet 113 is fixed to the outer circumferential surface of the voice coil bobbin 107. The movable magnet 113 is disposed between the cone-shaped diaphragm 105 and the damper 108, in the voice coil bobbin 107. The fixed magnet 114 is in the shape of a ring, and an inner circumferential surface of the fixed magnet 114 faces an outer circumferential surface of the movable magnet 113, forming a gap. The movable magnet 113 and the fixed magnet 114 are magnetized to the same polarity in a thickness direction (vibration direction).
Next, an operation of the conventional speaker device thus configured will be described. When an electrical signal is applied to the voice coil 109, a driving force is generated. The cone-shaped diaphragm 105 fixed to the voice coil bobbin 107 is vibrated by the driving force. Sound is generated by the vibration of the cone-shaped diaphragm 105. The above-described operation is an operation of a typical electrokinetic speaker. The conventional speaker device largely differs from typical speakers in an interaction between the movable magnet 113 fixed to the outer circumferential surface of the voice coil bobbin 107, and the fixed magnet 114 disposed facing the movable magnet 113. The cone-shaped diaphragm 105 is vibrated by the driving force generated by the voice coil 109. In this case, the movable magnet 113 is vibrated together with the voice coil bobbin 107 inside of the fixed magnet 114. The movable magnet 113 and the fixed magnet 114 are magnetized to the same polarity in the vibration direction. Therefore, when the movable magnet 113 is displaced, a magnetic field in which the movable magnet 113 and the fixed magnet 114 repel each other is formed. Therefore, when the movable magnet 113 is displaced from a position where the movable magnet 113 and the fixed magnet 114 are magnetically balanced (hereinafter referred to as a balanced position), a force which allows the movable magnet 113 to escape from the balanced position acts on the movable magnet 113. Specifically, the movable magnet 113 and the fixed magnet 114 act to apply a negative stiffness to a vibration system of the speaker unit 102 at a position deviated from the balanced position. In other words, the movable magnet 113 and the fixed magnet 114 constitute a mechanism for generating a negative stiffness. Hereinafter, the mechanism for generating a negative stiffness is referred to as a negative stiffness generating mechanism.
The negative stiffness acts on the vibration system of the speaker unit 102 so that an acoustic stiffness of the cabinet 101 is reduced. As a result, a minimum resonant frequency of the speaker device decreases. In other words, in the conventional speaker device, even the small-size cabinet can reproduce low-frequency sound as if the speaker unit were mounted in a large-size cabinet.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2000-308174
However, in the conventional speaker device, the movable magnet 113 which serves as a negative stiffness generating mechanism is provided in the voice coil bobbin 107. As a result, in the conventional speaker device, the weight of the vibration system of the speaker unit 102 increases, so that a level of an output sound pressure of the speaker unit 102 decreases.
In addition, if the size of the movable magnet 113 is reduced to decrease the weight of the vibration system, a magnetic field formed by the movable magnet 113 and the fixed magnet 114 is affected. Specifically, if the size of the movable magnet 113 is reduced, a force which gives the negative stiffness generated by the magnetic field decreases. Therefore, in the conventional speaker device, it is difficult to reduce the weight of the vibration system while keeping the force which gives the negative stiffness. Therefore, in order to increase the output sound pressure level of the speaker unit, it is conventionally necessary to reduce the weight of the vibration system and increase the driving force.